1. Field of the Invention
The present invention relates to a process for producing a spherical catalyst carrier, such as alumina spherical carrier, silica-alumina spherical carrier, zirconia-alumina spherical carrier, titania-alumina spherical carrier, boria-alumina spherical carrier, and boria-silica-alumina spherical carrier. This process permits easy production of a spherical carrier which has a pore structure almost identical to that of the major raw material such as alumina hydrate gel, silica-alumina hydrate gel, zirconia-alumina hydrate gel, titania-alumina hydrate gel, alumina hydrate paste, boria-alumina hydrate paste, and boria-silica alumina hydrate paste. The spherical catalyst carrier is composed of any of alumina, silica-alumina composition, zirconia-alumina composition, titania-alumina composition, boria-alumina composition, and boria-silica-alumina composition.
2. Description of the Related Art
A spherical carrier made of alumina is used as an adsorbent, catalyst, or catalyst carrier in the chemical industry and petrochemical industry. Also, a carrier of any of silica-alumina composition, zirconia-alumina composition, titania-alumina composition, boria-alumina composition, and boria-silica-alumina composition is used more extensively in the above-mentioned technical fields because of its larger specific surface area, better acid resistance and heat resistance, and higher solid acid concentrations as compared with the above-mentioned alumina carrier.
In general, a spherical carrier supporting a catalytic active metal is filled in a fixed-bed reactor or moving-bed reactor. Being spherical, it is uniformly filled in the reactor and easily removed from the reactor. Moreover, it does not cause channeling (a phenomenon that the reactant flows along a bypass during operation). It is also essential for reactions in a moving-bed reactor in which the reactant flows or moves downward by gravity.
Such a spherical carrier as mentioned above is produced by a "dropping in oil" process or a tumbling granulating process. According to the former process, an alumina spherical carrier is produced by gelling alumina sol in an alkaline atmosphere with hexamethylenetetramine or urea which decomposes at high temperatures to give off ammonia. The resulting spherical carrier has a smooth surface and is almost truly spherical. The gel made from alumina sol is homogeneous (from the surface to the inside) and has micropores with a diameter smaller than 50 .ANG..
The tumbling granulating process involves the steps of tumbling properly moistened alumina hydrate powder, thereby forming granules, and spraying water and supplying properly moistened alumina hydrate powder alternately, thereby thickening the layers and growing the granules. It yields a spherical carrier with a slightly rough surf ace and a broad distribution of sphericity. Granules formed by tumbling consist of layers and pores varying in size (ranging from macro to micro). Despite these disadvantages, it is widely used because of the low production cost.
A spherical carrier for a moving-bed reactor flows and/or moves during reaction and hence is required to have good wear resistance and a uniform, smooth surface. Therefore, it is produced mainly by the "dropping in oil" process. Despite its smooth surface and homogeneity, a spherical carrier made by the "dropping in oil" process contains micropores (smaller than 50 .ANG. in diameter) which account for a large portion. Such micropores are undesirable in some application areas, such as waste gas purification, in which physical poisoning and reduction in catalytic activity occur due to pore clogging.
In general, the performance of a catalyst is closely related with the pore characteristics of the carrier. Consequently, the carrier should have physical properties required of individual reaction conditions.